TS.5.3 Key vulnerabilities

Key vulnerabilities are found in many social, economic, biological and geophysical systems.

Vulnerability to climate change is the degree to which geophysical, biological and socio-economic systems are susceptible to, and unable to cope with, adverse impacts of climate change. The term “vulnerability” may therefore refer to the vulnerable system itself (e.g., low-lying islands or coastal cities), the impact to this system (e.g., flooding of coastal cities and agricultural lands or forced migration), or the mechanism causing these impacts (e.g., disintegration of the West Antarctic ice sheet). Based on a number of criteria in the literature (i.e., magnitude, timing, persistence/reversibility, potential for adaptation, distributional aspects, likelihood and ‘importance’ of the impacts [19.2]), some of these vulnerabilities might be identified as ‘key’. Key impacts and resultant key vulnerabilities are found in many social, economic, biological and geophysical systems [19.1.1].

The identification of potential key vulnerabilities is intended to provide guidance to decision-makers for identifying levels and rates of climate change that may be associated with ‘dangerous anthropogenic interference’ (DAI) with the climate system, in the terminology of the UNFCCC (United Nations Framework Convention on Climate Change) Article 2 [B19.1]. Ultimately, the determination of DAI cannot be based on scientific arguments alone, but involves other judgements informed by the state of scientific knowledge [19.1.1]. Table TS.8 presents an illustrative and selected list of key vulnerabilities.

Table TS.8. Table of selected key vulnerabilities. The key vulnerabilities range from those associated with societal systems, for which the adaptation potential is the greatest, to those associated with biophysical systems, which are likely to have the least adaptive capacity. Adaptation potential for key vulnerabilities resulting from extreme events is associated with the affected systems, most of which are socio-economic. Information is presented where available on how impacts may change at larger increases in global mean temperature (GMT). All increases in GMT are relative to circa 1990. Most impacts are the result of changes in climate, weather and/or sea level, not of temperature alone. In many cases climate change impacts are marginal or synergistic on top of other existing and possibly increasing stresses. Criteria for key vulnerabilities are given in Section TS 5.3. For full details refer to the corresponding text in Chapter 19. Confidence symbol legend: *** very high confidence, ** high confidence, * medium confidence, • low confidence.

Key systems or groups at risk

Prime criteria for ‘key vulnerability’

Global average temperature change above 1990

0°C

1°C

2°C

3°C

4°C

5°C

Global social systems

Food supply

Distribution, magnitude

Productivity decreases for some cereals in low latitudes **

Productivity increases for some cereals in mid/high latitudes **

Cereal productivity decreases in some mid/high latitude regions **

Global production potential increases to around 3°C, decreases above this * a

Aggregate market impacts and distribution

Magnitude, distribution

Net benefits in many high latitudes; net costs in many low latitudes * b

Benefits decrease, while costs increase. Net global cost * b

Regional system

Small islands

Irreversibility, magnitude, distribution, low adaptive capacity

Increasing coastal inundation and damage to infrastructure due to sea-level rise **

Key vulnerabilities may be linked to systemic thresholds where non-linear processes cause a system to shift from one major state to another (such as a hypothetical sudden change in the Asian monsoon or disintegration of the West Antarctic ice sheet or positive feedbacks from ecosystems switching from a sink to a source of CO2). Other key vulnerabilities can be associated with “normative thresholds” defined by stakeholders or decision-makers (e.g., a magnitude of sea-level rise no longer considered acceptable by low-lying coastal dwellers) [19.1.2].

Increasing levels of climate change will result in impacts associated with an increasing number of key vulnerabilities, and some key vulnerabilities have been associated with observed climate change.

Observed climate change to 2006 has been associated with some impacts that can be linked to key vulnerabilities. Among these are increases in human mortality during extreme weather events, and increasing problems associated with permafrost melting, glacier retreat and sea-level rise [19.3.2, 19.3.3, 19.3.4, 19.3.5, 19.3.6].

Global mean temperature changes of up to 2°C above 1990-2000 levels would exacerbate current key vulnerabilities, such as those listed above (high confidence), and cause others, such as reduced food security in many low-latitude nations (medium confidence). At the same time, some systems such as global agricultural productivity at mid- and high-latitudes, could benefit (medium confidence) [19.3.1, 19.3.2, 19.3.3].

Global mean temperature changes of 2 to 4°C above 1990-2000 levels would result in an increasing number of key impacts at all scales (high confidence), such as widespread loss of biodiversity, decreasing global agricultural productivity and commitment to widespread deglaciation of Greenland (high confidence) and West Antarctic (medium confidence) ice sheets [19.3.1, 19.3.4, 19.3.5].

Global mean temperature changes greater than 4°C above 1990-2000 levels would lead to major increases in vulnerability (very high confidence), exceeding the adaptive capacity of many systems (very high confidence) [19.3.1].

Regions already at high risk from observed climate variability and climate change are more likely to be adversely affected in the near future, due to projected changes in climate and increases in the magnitude and/or frequency of already damaging extreme events [19.3.6, 19.4.1].

The “reasons for concern” identified in the Third Assessment remain a viable framework to consider key vulnerabilities. Recent research has updated some of the findings from the Third Assessment.